it really depends on the level of the signals and the accuracy you need. the
no hardware way would be to read both seperately through seperate ADC
channels and subtract, if you wan't more accuracy or to save a pin on the
pic then build a differential amplifier (one op-amp and some resistors) or
an instrumentation amplifier (3 op-amps and more resistors but much higher
imput impedance).

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]On Behalf
> Of Chris Gavin-Egan
> Sent: 24 September 2006 00:15
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] Relative Newbie needs assistance
>
>
> Ok guess Jpgs aren't allowed... here's a link to the jpg
>
> http://www.gavin-egan.com/thermistor.jpg
>
> The full diagrams can be found at www.sinclairc5.com
>
> Chris Gavin-Egan wrote:
> > My ascii drawing didn't turn out too well, here's a jpg
> >
> >
> >
> > picture of circuit to ula
> >
> > Chris Gavin-Egan wrote:
> >
> >> Hi all
> >>
> >> I've been lurking for a while and I have been working on a
> hobby project
> >> for the past year (when i have had time) essentially learning
> whatever i
> >> can when I have spare time.
> >>
> >> Working on projects like this are the only way i find that i REALLY
> >> learn, however I think i am now truly stuck and quite possibly
> out of my
> >> depth and that is one of those thing i hate admitting.
> >>
> >> Basically I am working on producing a daughter board with a
> 16f628-A pic
> >> on it to replace an ageing 21 year old Ferranti ULA chip that is
> >> pre-programmed for working in an instrumentation pod of a Sinclair C5
> >> electric car. The problems associated with this is that I am working to
> >> the constraints of the original electronics that are there. I
> understand
> >> what the inputs and outputs of this chip are (which helps!!!)
> >>
> >> I am going to sell these to Sinclair enthusiasts when I finish, only
> >> because when the original chip malfunctions the C5 cant be driven. I
> >> will never make a profit with them because of the time and effort taken
> >> so far but in reality that is not the reason I am working on
> them. It is
> >> primarily just a project that forces me to learn but also because so
> >> many people over the years have made promises that they are
> working on a
> >> replacement chip for it and none have ever come to anything I dont want
> >> this project to fail and I am so close.
> >>
> >> The choice of 16f628 was primarily because of the I/O count ADC and the
> >> size of the chip in comparison to the ULA which is a 28pin DIL (Giving
> >> me a little room for manoeuvring within the original chip outline)
> >>
> >> I am about 60% of the way there. My daughter board works in a basic
> >> fashion - it operates the power control to the main power relay-
> >> illuminates LED's as appropriate and and i am just now trying to get
> >> some of the other functions to work.
> >>
> >> The First problem i have is really as follows.
> >> There is a 5v supply feeding a thermistor that is as shown below which
> >> detects the temperature of the motor across a std voltage divider .
> >>
> >>
> >> 5v------- 10k ntc thermistor------------------------- ferranti dil
> >> socket input 1
> >> | |
> >> | |
> >> | 100ohm
> >> 10nf |
> >> | |
> >> 0v-----------1k ohm------------------------------------------ferranti
> >> dil socket input 2
> >>
> >> It has a thermistor and voltage divider on 2 pins of the chip
> and what i
> >> am having problems with is understanding how to get pic to accept the
> >> voltage across these two pins.
> >>
> >> The operation of the system is that when the 10k thermistor reaches
> >> around 1000 ohms (in practice this is actually 750ohm )the input across
> >> the two is about 0.3 - 0.5v and the chip sets off an audible alarm i
> >> know it probably has a simple answer but my brain is now in a rut.
> >>
> >> What I was going to do is take the analogue voltage and display that on
> >> the LED bargraph as an approximate temp display. This display is
> >> presently being utilised by the ULA as a current meter for the motor, I
> >> simply thought that this display could be utilised as a temp display
> >> whenever the motor is not being used as well as utilising the audible
> >> output that is presently there.
> >> I hope this makes some sense.
> >>
> >> Its also funny how looking at this and talking about it on email has
> >> made me come up a couple of thoughts but my head hurts.
> >>
> >> Thankyou
> >>
> >>
> >>
> >>
> >>
> >>
> --
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